Vapor compression systems (VCS) in aircraft are used to supply refrigerant cooled air, which is used primarily for cooling avionic systems. The VCS is generally a closed-loop system, and may include a ram-air cooled condenser, an evaporator with a draw through fan, a centrifugal compressor driven by an electric motor, and a flash subcooler heat exchanger to improve overall system efficiency.
A low refrigerant charge level can result in decreased VCS system performance, particularly at higher heat loads. It may thus be appreciated that it is generally desirable to determine refrigerant charge level. However, the determination of refrigerant charge level can be difficult. This difficulty arises because the VCS operation is typically complex, with many interacting control loops. Generally the refrigerant charge level in the VCS is determined by indirect methods, such as inference from various system operating parameters. However, such methods to date have provided less than optimal results, for example due to a lack of a sufficient number of input variables, and/or due to substantial collinearity among input variables.
Accordingly, there is a need for an improved method and apparatus for determining refrigerant charge level in a VCS. The present invention addresses at least this need.